The technology relates to a developing device for use in an electrophotographic image forming apparatus of the type using a two-component developer comprising a toner and a carrier. More particularly, the technology relates to a developing device employing a counter developing system in which a developer bearing member feeds a developer at a developing region in a developer feeding direction which is opposite to an electrostatic latent image feeding direction in which an electrostatic latent image bearing member feeds an electrostatic latent image, as well as an image forming apparatus including such a developing device.
An electrophotographic image forming apparatus includes a developing device having a developer bearing member, and an electrostatic latent image bearing member for carrying an electrostatic latent image thereon. The developer bearing member supplies a developer to the peripheral surface of the electrostatic latent image bearing member at a developing region in which the electrostatic latent image bearing member and the developer bearing member face each other, to visualize the electrostatic latent image.
One developing system for such a developing device is a forward developing system in which the developer bearing member feeds the developer at the developing region in the same direction as the direction in which the electrostatic latent image bearing member feeds the electrostatic latent image, as described in Japanese Patent Laid-Open Publication No. H05-289522 for example.
With the forward developing system, when a high density region is developed in succession to development of a halftone region, a pinhole 201 is likely to occur in the halftone region as shown in FIG. 1. The pinhole 201 is considered to occur for the reason that electric flux lines 207 in a boundary area 206 of a halftone region 204 adjacent to a high density region 205 is deflected toward the high density region 205 as shown in FIG. 2 and, hence, an electric field is weakened in the boundary area 206.
With the forward developing system, after passage of the developer through the high density region 205, the amount of toner contained in the developer projecting like spikes is reduced, while an electric charge called “counter charge” appears at the tips of the spikes, whereby the developing ability of the developer is lowered. For this reason, the electrostatic latent image is more difficult to develop in the boundary area 206 in which the electric field is weakened, with the result that the pinhole 201 is likely to occur.
By contrast, with the counter developing system in which the developer feeding direction is opposite to the electrostatic latent image feeding direction of the electrostatic latent image bearing member, fresh spikes of the developer which have not passed through the developing region are applied to a downstream-side region of the electrostatic latent image bearing member 202 in the electrostatic latent image feeding direction. For this reason, the amount of toner contained in the developer is relatively large and the counter charge does not appear at the tips of the spikes. Therefore, the developing ability of the developer is relatively high in the boundary area 206, so that the pinhole 201 is less likely to occur in the halftone region 204 developed prior to the high density region 205.
With the counter developing system, however, a phenomenon so-called “sweeping together” that a trailing end portion of an image (i.e., an upstream end portion of the electrostatic latent image in the electrostatic latent image feeding direction) is developed excessively densely, is likely to occur.
The reason why such a phenomenon occurs is as follows. On the side upstream of a proximal position at which the electrostatic latent image bearing member and the developer bearing member are closest to each other in the direction of rotation of the developer bearing member, the developer projecting like spikes has not passed through the proximal position yet and, hence, spikes of the developer are relatively high. For this reason, the developer collides with the peripheral surface of the electrostatic latent image bearing member to cause toner particles to float. In the trailing end portion of the image located adjacent to an image-free portion, the density of electric flux lines directed from the peripheral surface of the developer bearing member toward the peripheral surface of the electrostatic latent image bearing member is likely to increase and, hence, floating toner particles collect easily. Further, since the side upstream of the proximal position at which the developer bearing member is closest to the electrostatic latent image in the developer feeding direction corresponds to the side downstream of the proximal position in the electrostatic latent image feeding direction, the quality of the image tends to be determined on the side downstream of the proximal position in the electrostatic latent image feeding direction. In a conventional image forming apparatus employing the counter developing system, floating toner particles are likely to collect on the trailing end portion of an image in a region in which the image quality tends to be determined.
The technology intends to provide a developing device which is capable of suppressing the occurrence of a pinhole in the halftone region developed prior to the high density region, as well as the occurrence of the phenomenon that the trailing end portion of an image becomes excessively dense.